Development of hydrophobic and optically transparent monolithic silica aerogels for window panel applications

In the present paper, highly transparent, monolithic and hydrophobic silica aerogel monoliths were prepared by using tetraethoxysilane (TEOS) as precursor and methyltriethoxysilane (MTES) as co-precursor with different MTES/TEOS molar ratios (M) by two-step acid–base catalyzed sol–gel process followed by supercritical alcohol drying. The molar ratio of TEOS, ethanol (EtOH), water (0.001 M oxalic acid catalyst) and ammonium hydroxide (1 M NH₄OH) was kept constant at 1:5:3.5:3.5 respectively, while the molar ratio of MTES/TEOS (M) was varied from 0 to 0.75. It has been observed that as the M value increases, the gelation time increases. It has been found that lower (0.25) M values resulted in highly transparent (optical transmission >90 % for a 10 mm thick sample at 700 nm wavelength) and negligible volume shrinkage (<10 %) but less hydrophobic aerogels, whereas higher (0.75) M values resulted in semitransparent (<25 % optical transmission at 700 nm for a 10 mm thick sample) aerogels with >10 % volume shrinkage but excellent hydrophobicity. A good compromise of acceptable optical transmittance (~87 % optical transmission at 700 nm wavelength for a 10 mm thick sample) with negligible volume shrinkage (6 %) were obtained at M = 0.50. The hydrophobicity of the aerogels was tested by measuring the contact angle between a water droplet and silica aerogel surface. The hydrophobicity was confirmed by Fourier transform infra-red spectroscopy and contact angle measurements. the aerogels have been characterized by percentage of volume shrinkage, optical transmittance, scanning electron microscopy, thermal conductivity measurements and thermogravimetric analysis and differential thermal analysis.     

有机-无机杂化柔性硅气凝胶的制备与表征

以甲基三甲氧基硅烷（MTMS）和四乙氧基硅烷（TEOS）为混合硅源、甲醇为溶剂,通过酸碱两步催化溶胶-凝胶法制备湿凝胶,经超临界流体干燥得到块状二氧化硅气凝胶。用扫描电镜、氮气吸附脱附测试以及热重分析等手段对气凝胶的微观形貌、比表面积、孔径分布、弯曲性、压缩性、热稳定性等进行研究,结果表明：MTMS/TEOS比例会影响气凝胶的微观结构、弯曲和压缩性以及热稳定性,以MTMS/TEOS=8/1制得的气凝胶密度为0.11 g·cm^-3、孔隙率为94.2%、比表面积为693.3 m²·g^-1、最大弯曲角可达92°、最大压缩比例可达41.2%、压缩回弹率为100%。     

酸碱催化剂浓度对柔性硅气凝胶性能和结构的影响

以甲基三甲氧基硅烷(MTMS)和正硅酸乙酯(TEOS)作为混合硅源,甲醇为溶剂,十六烷基三甲基溴化铵(CTAB)为表面活性剂,通过酸碱两步催化溶胶凝胶法制备醇凝胶,经超临界干燥可以制备高弹性疏水块状硅气凝胶。分别采用0.1 mol·L^-1 和0.01 mol·L^-1 的草酸作为酸催化剂,5 mol·L^-1 和10 mol·L^-1 的氨水作为碱催化剂,研究不同酸碱催化剂浓度对其网络结构的影响。发现高浓度酸和高浓度碱以及低浓度酸和低浓度碱作为催化剂合成的硅气凝胶的网络结构更加均匀,孔径分布更窄。其中,在草酸浓度为0.01 mol·L^-1、氨水浓度为5mol·L^-1 时,所得硅气凝胶密度为0.135 g·cm^-3、比表面积为807 m²·g^-1、孔隙率约为93%,凝胶最大可压缩至其起始长度的60%,压缩回弹率为100%。     

In situ production of spherical aerogel microparticles

Due to their high surface area, low density, open pore structure and excellent insulation properties aerogels are intensively investigated since the past decades for a diverse range of applications. The current methods of silica aerogel production by supercritical extraction produce monolithic aerogels, where the sol is aged in molds and dried by extraction with supercritical CO₂. Aerogels in the form of spherical microparticles would be beneficial for many applications, for instance, drug delivery for respiratory route; or as insulating materials. However, because of aerogel's mechanical properties, it is difficult, rather impossible, to obtain spherical microparticles by milling or crushing of the monolithic aerogels. This work presents a new method to produce biocompatible spherical aerogel microparticles using an emulsion technique (in situ production) followed by supercritical extraction of the resulted dispersion (gel-oil). Water in oil emulsion was produced by mixing the sol (dispersed phase) with a vegetable oil (continuous phase) followed by the gelation of the dispersed phase. The size distribution of the final gel particles was found to be influenced by agitation, surfactant concentration and sol:oil volume ratios. The gel-oil dispersion was subsequently extracted with supercritical CO₂, Silica aerogel spherical microparticles with a surface area of 1100 m²g⁻¹, pore volume of 3.5 cm³/g and different mean particle diameters ranging from 200 μm to a few millimeters were produced using the presented method.     

疏水性二氧化硅气凝胶的常压制备与表征

采用溶胶-凝胶技术,以正硅酸乙酯为有机前驱体,三甲基氯硅烷为改性剂,在常压下干燥制备出疏水性二氧化硅(SiO2)气凝胶,同时采用密度测量、比表面积和孔径分布仪、扫描电镜、热重分析仪对SiO2气凝胶进行表征。文章重点研究反应物配比和表面修饰对SiO2气凝胶性能的影响。结果表明,采用正硅酸乙酯、水、乙醇体积配比为25︰6︰80和三甲基氯硅烷的正己烷溶液表面修饰合成的SiO2气凝胶性能质量较好。     

Effect of aluminium and copper acetylacetonate on physico-chemical properties of tetraethoxysilane based silica aerogels

Silica aerogel granules have been synthesized by doping aluminium acetylacetonate (Al(acac)₃) and Copper acetylacetonate (Cu(acac)₂), in the sol of tetraethoxysilane followed by ambient pressure drying process. The well established silica network provides effective confinement for metal particles which resists the collapse of silica network during ambient pressure drying of wet gel. In this paper, the effect of Al(acac)₃ and Cu(acac)₂ on physico-chemical properties of the aerogels have been studied by varying the concentration of both from 0.0005 to 0.05 M. The chemical composition, hydrophobicity and elemental analysis of Al(acac)₃ and Cu(acac)₂ doped silica aerogels were studied by using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, contact angle measurement, X-ray diffraction, atomic absorption spectroscopy. The morphological study was carried by using transmission electron microscopy and surface area and pore diameter was measured by BET analysis. The thermal stability of hydrophobic nature was analyzed by simultaneous thermogravimetry and differential scanning calorimetry. The Al(acac)₃ added silica aerogels are found to be superhydrophobic, low dense, high surface area and optically transparent as compared with Cu(acac)₂ doped silica aerogels.     

Effect of sodium bicarbonate solution on methyltrimethoxysilane-derived silica aerogels dried at ambient pressure

Here we present an economical ambient pressure drying method of preparing monolithic silica aerogels from methyltrimethoxysilane precursor while using sodium bicarbonate solution as the exchanging solvent. We prepared silica aerogels with a density and a specific surface area of 0.053 g∙cm⁻³ and 423 m²∙g⁻¹, respectively. The average pore diameter of silica aerogels is 23 nm as the pore specific volume is 1.11 cm³∙g⁻¹. Further, the contact angle between water droplet and the surface of silica aerogels in specific condition can be as high as 166°, which indicates a super-hydrophobic surface of aerogels.     

Alumina/silica aerogel with zinc chloride alkylation catalyst: Influence of supercritical drying conditions and aerogel structure on alkylation catalytic activity

Alumina/silica aerogel with zinc chloride alkylation catalyst, was obtained using one step sol–gel synthesis and subsequent drying with the supercritical carbon dioxide. The high density supercritical carbon dioxide drying conditions resulted in zinc chloride removal from the catalyst surface, surface area and pore volume increase and catalytic activity decrease. The low density supercritical carbon dioxide drying conditions, pore size distribution centred around 6 nm pore radius and high degree of mixed Al–O–Si bonds in the alumina/silica aerogel network, were found to increase the catalytic activity of the obtained aerogel catalysts.     

低密度SiO_2气凝胶的制备及性质表征

通过二步溶胶－凝胶法和ＳＣＦＤ过程制备了低密度ＳｉＯ２气凝胶,并应用ＳＥＭ,自动吸附仪,元素分析仪对气凝胶样品进行了结构和性质表征,且分析了ＴＥＯＳ浓度和Ｈ２Ｏ／ＴＥＯＳ值对气凝胶结构和性质的影响。结果表明,用此方法可以制备密度低至０．０２ｇ·ｃｍ－３的ＳｉＯ２气凝胶,具有较高比表面积和孔体积,充满两端开放的管状中孔（ｒｐ＝１３．６ｎｍ）,且含有少量以物理和化学状态结合的碳与氢。     

常压制备低密度SiO2气凝胶的正交分析及表征

选择5因素4水平的L16(45)正交试验,以正硅酸乙酯(tetraethoxysilane,TEOS)为原料,采用酸碱两步催化溶胶-凝胶法和常压干燥制备了SiO2气凝胶,分析了正交实验因素对结果的影响,并对气凝胶的形貌、表面基团、疏水性、孔结构进行了表征.实验结果表明:表面改性对制备低密度SiO2气凝胶影响最大,正交试...     

常压干燥法制备疏水性SiO2气凝胶

利用正硅酸乙酯做先躯体,盐酸与氨水做催化剂,通过二步法制备了二氧化硅气凝胶,并利用三甲基氯硅烷和正己烷做表面改性剂采用共沸法逐级对湿凝胶进行表面改性.测试分析了气凝胶的密度、表观、疏水性、比表面积、孔径分布和热稳定性.结果表明:气凝胶的物理特性及其疏水性受到三甲基氯硅烷与正己烷的体积比(ψ)的影响,其密度随着ψ增大而减小,当ψ=3%时,气凝胶的疏水性最好;所制得的SiO2在350℃仍具有良好的热稳定性,并且具有高比表面积(838.6 m2/g)和孔体积(2.10cm3/g).     

Synthesis and Characterization of Hydrophobic TMES/TEOS Based Silica Aerogels

The experimental results on the effect of adding trimethylethoxysilane (TMES) as a co-precursor on the hydrophobicity and physical properties of tetraethoxysilane (TEOS) based silica aerogels, are reported. The molar ratio of TEOS, ethanol (EtOH), water (0.001 M oxalic acid catalyst) was kept constant at 1:5:7 respectively, while the molar ratio of TMES/TEOS (A) was varied from 0 to 0.6. It has been observed that as the A value increases, the gelation time increases. The hydrophobicity was tested by measuring the contact angle, and the surface chemical modification was confirmed by the FTIR spectroscopy studies. The thermal stability of the hydrophobic aerogels was studied in the temperature range from 25 to 800°C. The hydrophobic nature of the aerogel could be maintained up to a temperature of 287°C and above this temperature the aerogels become hydrophilic. The bulk density and the optical transmittance of the aerogels have been found to decrease with increase in A value. The aerogels have been characterized by Fourier transform infrared spectroscopy (FTIR), Optical transmittance, Scanning electron microscopy (SEM), Differential thermal analysis (DTA) and Thermogravimetric analysis (TGA), and Contact angle measurements.     

Methyltrimethoxysilane based monolithic silica aerogels via ambient pressure drying

We have developed a novel route to monolithic silica aerogels via ambient pressure drying by the acid–base sol–gel polymerization of methyltrimethoxysilane (MTMS) precursor. An extent of silica polymerization in the alcogels plays a crucial role in obtaining the monolithic aerogels which could be optimized by a proper control over the MeOH/MTMS molar ratio (S) during the sol–gel synthesis. The alcogel undergoes the distinct “spring-back effect” at the critical stage of the drying and thereby preserving the highly porous silica network without collapse. The process yields silica aerogels exhibiting very low bulk density and high specific surface area of 0.062 g/cm³ and 520 m²/g, respectively. The average pore diameter and the cumulative pore volume varied from 4.5 to 12.1 nm and 0.58 to 1.58 cc/g, respectively. In addition, the aerogels are superhydrophobic with contact angle as high as 152°. We anticipate that the new route of the monolithic silica aerogel production will greatly expand the commercial exploitation of these materials.     

纳米孔超级绝热材料气凝胶的制备与热学特性

以正硅酸四乙酯(TEOS)为硅源，通过溶胶–凝胶及超临界干燥过程制备了SiO2气凝胶. 同时，采用相对廉价的多聚硅(E–40)为硅源，以三甲基氯硅烷(TMCS)为表面修饰剂，硅油为干燥介质, 在常压条件下制备了同样具有纳米多孔结构的SiO2气凝胶. 用透射电镜、扫描电镜及孔径分布仪对其结构进行了表征，并用动态热线法对其热学特性进行了测试. 结果表明: 两种方法制备的气凝胶均是典型的纳米孔超级绝热材料，后者热导率略高但成本低许多，所以更具应用推广潜力.     

超临界干燥制备疏水型二氧化硅气凝胶

文章以正硅酸乙酯为原料,经溶胶-凝胶过程制备二氧化硅醇凝胶,采用三甲基氯硅烷作为表面改性剂,对醇凝胶进行化学表面修饰,超临界干燥,制备了疏水性二氧化硅气凝胶粉末。运用红外光谱、BET、扫描电镜、XRD对其结构、形貌及化学组成进行了分析。结果表明：该样品是表面连有疏水基团-CH3的疏水性SiO2气凝胶,呈连续网络结构的球状纳米粒子,孔径分布主要集中在2～4nm,是热稳定性较高的非晶、多孔、轻质介孔材料。     

溶胶凝胶一步溶剂交换与表面改性制备疏水硅气凝胶

以正硅酸乙酯为硅源,采用酸碱两步催化的溶胶-凝胶法,以无水乙醇/六甲基二硅氮烷/正己烷为溶剂交换与表面改性试剂,通过一步溶剂交换与表面改性和常压干燥工艺制备疏水SiO2气凝胶。用傅立叶变换红外光谱(FTIR)、X射线衍射(XRD)、热重-差示(TG-DSC)、场发射扫描电镜(F-SEM)和比表面积(BET)等检测手段对样品的结构、形貌和性能进行了表征。结果表明,将传统的溶剂交换与表面改性由多个步骤改为一步完成,使制备周期从原来的6d缩减到3d,不仅缩短了周期,而且使所得的SiO2气凝胶样品具有842.63m2/g的高比表面积、130kg/m3的低密度、15nm的超细颗粒、2～170nm的孔洞结构和良好的疏水性能。     

Influence of aging conditions on textural properties of water-glass-based silica aerogels prepared at ambient pressure

The experimental results of aging time and temperature on the textural properties of water-glass (sodium silicate)-based silica aerogels are reported and discussed. Aging of the hydrogel for different times and temperatures led to an ability to increase the stiffness and strength of the networks. These improvements enabled the gel to withstand ambient pressure drying (APD) and, consequently, preserve the highly porous silica network without collapse. The pore size and volume increased with increasing aging temperature and time, while the specific surface area decreased. Monolithic aerogels with extremely low bulk density (～0.069 g/cm³), high specific surface area (820 m²g^?1), large cumulative pore volume (3.8 cm³g^?1), and high porosity (～96%) were obtained by aging at 60 °C for 18 hours. Therefore, easy synthesis of monolithic silica aerogels at ambient pressure is achievable using a relatively inexpensive silica precursor (sodium silicate).     

Two step sol-gel processing of TEOS based hydrophobic silica aerogels using trimethylethoxysilane as a co-precursor

The experimental results on the surface modification of tetraethoxysilane (TEOS) based silica aerogels using trimethylethoxysilane (TMES) as a co-precursor by two step sol-gel process are reported. The molar ratio of MeOH/TEOS was fixed at 17 and TMES/TEOS ratio was varied from 0.38 to 1.14. The concentration and quantity of acidic water (oxalic acid) added in the first step and base water (ammonium hydroxide) in the second step were also varied. The best quality superhydrophobic aerogels could be obtained with only distilled water (without any acid catalyst) in the first stage and the basic water in the second step of the sol-gel process. The molar ratio of TEOS: TMES: MeOH: distilled water: basic water was optimized at 1:0.86:17:4.9:4.9, respectively. The surface modification has been confirmed from the Fourier transform infrared spectroscopy measurements while the hydrophobicity was quantified in terms of the contact angle measurements. The TMES/TEOS based aerogel powder could be used to transport micro-liters of water in the form of water marbles. The aerogels have been characterized by the bulk density, porosity, thermal conductivity, contact angle measurements and the transmission electron microscopy (TEM).      

二氧化硅气凝胶的制备和表征

以正硅酸四乙酯为硅源，通过采取老化、表面修饰、溶剂置换和分级干燥等一系列抑制二氧化硅气凝胶干燥中出现缩裂的有效措施，以非超临界干燥技术最终获得了大块无裂纹的二氧化硅气凝胶。该气凝胶的孔径较小且分布均匀，比表面积为684m^2／g，孔体积可达1．38cm^3／g，最可几孔径为3．221nm，平均孔径达2．871m。同时在实验和理论分析的基础上总结二氧化硅气凝胶缩裂的主要原因和抑制缩裂的有效措施。     

SiO2气凝胶的非超临界干燥法制备与表征

以聚二甲基硅烷为原料,采用聚合物超临界法制备了SiO2气凝胶.聚合物超临界法制备的SiO2气凝胶除碳前为疏水性气凝胶,比表面积为27.68 m2/g,孔体积为0.103 7 cm3/g,平均孔径约为15 nm;除碳后为亲水性气凝胶,比表面积为500.6 m2/g,孔体积为0.404 3 cm3/g,平均孔径为3.23 nm.对聚合物超临界法制备SiO2气凝胶的反应机理进行了初步探讨.